Dielectric breakdown occurs when the electric field becomes

Dielectric breakdown occurs when the electric field becomes greater than an insulator (dielectric) can withstand, and a spark will go through the dielectric. For air, breakdown occurs when the electric field strength is ( approximately) 30,000 V/cm and a spark or arc is formed. sally sue, after walking

Solution

The voltage in the gap of V = E*d ; E is the electric field and d is the air gap

                                           = 30,000 V/ cm * 1.5 cm = 45000 V

if the amount of energy in the spark E = 2.5 * 10^-4 J

             we know E = V * Q   ; Q is the total charge tranferd

                            Q = V /E = 45000 / 2.5 * 10^-4 J

                                          = 1.8 * 10 ^8 C

      we know each electron has charge q = 1.6 *10^-19 C

     if N be the Number of electrons in charge Q is    N = Q /q     = 1.8 * 10 ^8 C / 1.6 *10^-19 C /electron

                                                                                     N = 1.125 * 10^27 electrons